The present invention relates to the field of injection-moulding technology. It concerns an injection-moulding tool for the production of information carriers in disc form.
Such an injection-moulding tool is known, for example, from the printed document WO-A1-99/37471.
For producing optically readable information carriers such as audio CDs, CD-ROMs, video CDs or DVDs, injection-moulding tools which form an injection mould by means of two (cylindrical) mirror blocks lying opposite each other and movable in relation to each other are used. On one of the two mirror blocks there is usually releasably fastened a thin, disc-shaped stamper, which stamps the tracks containing the information during the injection moulding of the information carrier being formed. At the outer edge, the injection mould is often bounded by a form ring, which concentrically surrounds one of the mirror blocks.
The form ring projects beyond the planar moulding surface of the mirror block enclosed by it and is displaceable in relation to this mirror block in the axial direction. If the injection mould is closed by an axial movement of the two mirror blocks towards each other, the form ring comes to rest on the opposite mirror block on account of the projection and closes the mould, while the mirror block concentrically enclosed by it is at a predetermined distance from the opposite mirror block. The hot plastic is then infected under high pressure into the disc-shaped cavity formed in this way. Once the cavity of the injection mould has been filled, the mirror block enclosed by the form ring is moved a little towards the other mirror blockxe2x80x94in order to achieve a high quality of the information carrierxe2x80x94, while the form ring remains fixed in place (see in this respect FIGS. 2 and 3 of WO-A1-99/37471, cited at the beginning).
However, the relative movement taking place in this phase of the production process between the mirror block and the form ring enclosing it gives rise to problems for the following reasons: in order that the form ring can slide freely over the mirror block during the relative movement, a certain fit is necessary. If this fit is too large, plastic can get into the gap during injection and forms undesired flash on the moulded information carrier disc. If, on the other hand, the fit is too close, excessive friction values may occur, in particular if the mirror block and form ring are at different temperatures, hindering the relative movement andxe2x80x94because both parts are generally made of steelxe2x80x94possibly leading to seizing or jamming of the form ring on the mirror block.
In order to ensure a low sliding friction between the mirror block and the form ring when there is a relatively close fit, it has already been proposed in WO-A1-99/37471, cited at the beginning, to provide the surfaces sliding on one another of the mirror block and the form ring with a special coating which reduces the friction. However, it is disadvantageous in this case that the coating of the parts entails high expenditure, and that the coating generally influences the heat removal from the injection mould.
In EP-A1-0 899 075, to ensure a good relative mobility between a mirror block and a ring concentrically enclosing the mirror block, a linear ball bearing (75 in FIG. 5) is provided between the two parts. However, in this case the ring is not a form ring, which forms part of the injection mould, but a guide ring, which assists the centering of the two mirror blocks with respect to each other.
Finally, U.S. Pat. No. 5,018,962 discloses an injection-moulding tool, comprising two mirror blocks and a form ring, for the production of CDs, in which a concentric retaining ring (15 in FIG. 1) for the stamper resting on one of the mirror blocks is provided between the form ring and the said mirror block. During opening and closing, the form ring slides over both mirror blocks. However, special precautions for reducing the friction during these sliding processes are not taken.
It is therefore the object of the invention to provide an injection-moulding tool for the production of information carriers in disc form which avoids the disadvantages of known injection-moulding tools and is distinguished in particular by simplified production, can be used unproblematically and flexibly and makes possible the production of information carrier discs of a particularly high quality.
The object of the invention is achieved by the injection moulding tool comprising an injection mould, a first and a second block, which lie opposite each mould, and a form ring, which concentrically encloses the first edge and is movable in relation to the first mirror block, means for reducing the friction when there is a movement of the form ring in relation to the first mirror block being provided between the first mirror block and the form ring enclosing it, characterized in that the means comprise a sliding ring arranged concentrically between the first mirror block and the form ring enclosing it. The essence of the invention is to provide between the parts sliding over one another of the injection-moulding tool, to be specific the first mirror block and the form ring enclosing it, a special sliding ring which prevents direct friction between the mirror block and the form ring. Since the sliding ring is a separate component, if need be it can easily be exchanged or replaced, without having to change the change the other parts of the injection-moulding tool; in a corresponding way, a free choice of the material of the sliding ring allows the sliding properties to be optimized. Since the sliding ring inevitably forms part of the injection mould at the edge, the form of the injection-moulded part can be influenced by suitably shaping and selection. Finally, it is possible by a specific choice of the material of the sliding ring, to improve the heat removal in the edge region of the injection mould.
A first preferred embodiment of the injection-moulding tool according to the invention is distinguished in that the sliding ring consists of a material reducing the sliding friction, in particular of copper, a copper alloy such as bronze or else a plastics material such as Teflon. As a result, equally good sliding properties and good thermal conductivities can be realized in the edge region of the injection mould. The sliding ring can also be provided with a friction-reducing coating, for example a PVD coating (PVD=physical vapour deposit).
Influencing the form of the injection-moulded part by corresponding shaping of the sliding ring according to the invention can be achieved in various ways: if the first mirror block bounds the injection mould with an essentially planar surface, and the sliding ring adjoins the planar surface of the first mirror block with its end face facing the injection mould in such a way that it is flush with it, and enlarges it towards the outside, an injection-moulded part with a planar underside up to the outer edge is obtained.
If, on the other hand, the sliding ring has a bevel on its end face facing the injection mould in such a way that the information carrier produced in the injection mould has a chamfer at its outer edge, the injection-moulded part is correspondingly provided with a chamfer.
If, finally, the sliding ring has a peripheral depression on its end face facing the injection mould in such a way that the information carrier produced in the injection mould has an elevated, annular scratch guard at its outer edge, the injection-moulded part is correspondingly provided with a scratch guard in the form of an annular bead.
Further embodiments emerge from the dependent claims.